Here we go again. Yet another heatwave here in London. And, with the Met Office forecasting 34C, it’s very likely we’ll see a daily record for 17th June (beating 33.9C recorded at Little Massingham in 1917, according to my trusty source, The Wrong Kind of Snow, by Antony Woodward and Robert Penn) and possibly yet another UK “date record”, the term I’ve coined for the highest (or lowest) temperature so early or late in the year. For that to happen, the temperature would have to exceed the rather freakish 34.4C recorded at Waddington on 3rd(!) June 1947. Anything higher than the 34.5C recorded on 21st June 2017 would be the hottest earlier than 26th June, when 35.4C was recorded in 1976, followed by 35.5C on 27th and 35.6C on 28th.
Clearly, more frequent heatwaves is one of the more immediately problematic effects of global warming.
What is rarely discussed is exactly why heatwaves have become so much more severe.
The point is that, outside the tropics, heatwaves are due to the movement of air towards higher latitudes:
Now, global warming (GW) arises not because more of the Sun’s heat reaches the Earth’s surface. No, no, no. It arises because heat is radiated away less efficiently. Some of it is captured by greenhouse gases – carbon dioxide, methane, nitrous oxide etc – and retained for longer in the atmosphere than it was before humans foolishly raised the concentrations of those gases.
But this increased heat retention in the atmosphere doesn’t just happen instantaneously. It happens continually. Let’s consider how this might be affecting the current London heatwave.
The hot air initially originated over Africa. Because of GW it lost less heat than it would have done previously whilst still over that continent. But it would only have been warmer than before GW by approximately the average increase in temperatures over land due to GW – say ~1.5C.
The problem is that the airmass then moved north to Spain and southern France, causing a notable heatwave. Why? Because not only did it start off hotter than it would have done before GW, it also lost less heat on the way, because it is radiating it away less efficiently.
Then, as pointed out by Liam Dutton (see Fig 1), some of the hot air over France moved towards the UK, again losing less heat on the way than it would have done absent GW.
Thus, with fairly unexceptional conditions – no real build-up of heat with drying out of the soils reducing evaporative cooling and so on, just a wisp of Saharan air reaching London – we have potentially record-breaking temperatures here.
Given that a feature of the Earth’s climate system (and that of any planet with an atmosphere and/or ocean) is that there is net heat transport towards the poles, heat transport thus magnifies GW at higher latitudes.
Computer models show increased heating at high latitudes, of course, but it rather frustrates me that the conceptual point – that initially warmer air because of GW loses less heat than before GW as it moves towards the Earth’s poles – is never explained. Instead the albedo effect is almost always invoked, as, for example, in this discussion of the alarming warming of the Barents Sea:
Sea ice is good at reflecting sunlight but is melting away. This allows the darker ocean below to absorb more energy. Losing sea ice also means it no longer restricts the ability of warmer sea waters to heat up the Arctic air. The more ice is lost, the more heat accumulates, forming a feedback loop.
“We expected to see strong warming, but not on the scale we found,” said Ketil Isaksen, senior researched at the Norwegian Meteorological Institute and who led the work. “We were all surprised. From what we know from all other observation points on the globe, these are the highest warming rates we have observed so far.”
“The broader message is that the feedback of melting sea ice is even higher than previously shown,” he said. “This is an early warning for what’s happening in the rest of the Arctic if this melting continues, and what is most likely to happen in the next decades.”excerpt from Guardian article New data reveals extraordinary global heating in the Arctic, 14/6/22
I venture to suggest that “the feedback of melting sea ice” isn’t the whole story.